Michael Schumacher's condition remains stable but critical and he is under constant monitoring at the Grenoble hospital following a skiing accident on Dec 29, his manager has confirmed.

The seven-time Formula One world champion has undergone two operations as neurosurgeons removed blood clots and reduced swelling, with the 45-year-old in an artificially induced coma.

Schumacher suffered the head injury whilst on a family holiday in the French resort of Meribel and, despite being conscious immediately after the accident, he was airlifted to hospital where the full extent of his injuries became clear.

Since then he has been declared as stable and his manager Sabine Kehm has said his condition has not changed and any treatment will be kept private.

"The clinical state of Michael Schumacher is considered as stable and is being continually monitored by the medical treatment administered to him," Kehm said in a statement.

"However, the medical team in charge emphasise that they continue to judge his condition critical.

"The patient's privacy requires that we do not give out details of his treatment and it is for this reason that we do not envisage any new press conferences, nor to send out any written press releases, in the near future."

Supporters gathered outside the hospital on Friday as Schumacher turned 45 but it still remains unclear if the former Jordan, Benetton, Ferrari and Mercedes driver will fully recover from the injuries sustained.